Formaldehyde (FA) is one of the most important commodity chemicals; the production in the EU in 2002 was more than 3 million tonnes; double the amount is likely to have been produced worldwide (5.3 million tonnes (1989)) [K. Weissermel, H.-J. Arpe, Industrielle Organische Chemie, VCH, 1994]. FA is required especially for the production of various resins (phenol, melamine, etc.) and polymers. The synthesis proceeds virtually exclusively from methanol [K. Weissermel, H.-J. Arpe, Industrielle Organische Chemie, VCH, 1994; M. Qian, M. A. Liauw, G. Emig, Appl. Catal. A: General, 238 (2003) 211], although the direct synthesis from methane has also been studied [H. Berndt, A. Martin, A. Brückner, E. Schreier, D. Müller, H. Kosslick, G.-U. Wolf, B. Lücke, J. Catal., 191 (2000) 384]. The synthesis from methanol is effected either by dehydrogenation or partial oxidation. While the dehydrogenation (Eq. 1) proceeds endothermically (+84 kJ/mol), the partial oxidation (Eq. 2) is strongly exothermic (−159 kJ/mol).CH3OHHCHO+H2  Eq. 1CH3OH+0.5O2→HCHO+H2O  Eq. 2
The partial oxidation is performed in two different processes with different catalysts. The main distinguishing features are firstly the different catalysts but secondly the observance of the explosion limits of MeOH in air.
In the air deficiency or silver contact process (BASF, Borden, Bayer, Degussa, ICI, Celanese, DuPont, Mitsubishi, Mitsui, plant sizes: 80-135 kt/a of FA), MeOH contents of >37.5% by volume are employed. In the air excess, molybdate or FORMOX process (Lummus, Montecatini, Hiag/Lurgi, Perstorp/Reichsdorf, plant sizes: 20-30 kt/a of FA), the process is conducted at MeOH contents of <7% by volume. About 80% of all plants operate by the air deficiency or silver contact process; approx. 55% of FA production in Western Europe is based on the silver contact process [M. Qian, M. A. Liauw, G. Emig, Appl. Catal. A: General, 238 (2003) 211; Catalysis from A to Z, VCH Wiley, 2000, p. 224].
Partial oxidation by the air deficiency process is performed principally in two variants: (i) MeOH ballast process (e.g. Degussa, ICI) and (ii) the water ballast process (BASF). Whereas an incomplete conversion is achieved in the first process, in which only MeOH and air are used, a virtually full conversion can be achieved in the second process with additional metering of steam. It is normal to operate with an MeOH/H2O mixture of 60/40.
The currently available processes, however, are all affected by the fact that the silver catalyst is caking relatively rapidly at the reaction temperatures and it becomes ever more difficult for the gases to be introduced to flow through the catalyst bed. When the expenditure at this point becomes too great, the formaldehyde plant has to be shut down and the catalyst replaced, which leads to costly production shutdowns. The high heat capacity of water achieves homogeneous distribution of heat in the water ballast process and protects the catalyst from excessively rapid “sintering”. Moreover, the steam helps to prevent or to minimize coke formation. For these reasons, the lifetime of the silver catalyst in the water ballast process is significantly higher than in the methanol ballast process, though a further increase in service life would be desirable (WO 0130492)
It is known in principle to produce nanoparticulate silver-platinum alloys which have a coating of porous silica gel (Ultra-thin porous silica coated silver-platinum alloy nano-particle as a new catalyst precursor, Kai Man K. Yu, David Thompsett, Shik Chi Tsang, Chem. Commun., 2003, (13), 1522-1523). However, such nanoparticles are unsuitable for formaldehyde synthesis, since they have much too high a bulk density and it would thus be very difficult for the gases to be converted to flow through them when used in the catalyst bed.